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Working principle of spray dryer

Spray dryer is a modern high-efficiency drying equipment, which is widely used in food, pharmaceutical and chemical industries. Its working principle involves multiple key stages, including raw material liquid atomization, hot air generation, mixed contact drying, evaporation to form powder, drying stage division, material air separation, fine material collection, and exhaust gas emission treatment. The following is a detailed explanation of these stages:

Atomization of raw liquid is the first step of spray drying. At this stage, the raw liquid (which can be solution, emulsion, suspension, or molten liquid or paste and other liquid that can be transported by pump) is sent to the top of the spray dryer through the peristaltic pump or other conveying equipment. Then, use an atomizer to disperse the raw material liquid into tiny droplets. The atomizer converts the raw material liquid into fine water droplets through a specific nozzle, forming a mist. The purpose of atomization is to increase the surface area of the raw material liquid, so as to quickly evaporate moisture when in contact with hot air.

In a spray dryer, the air is first purified through a filter to remove dust and impurities. Then, it is heated by a heater to the specified temperature. The heated air is sent into the drying tower in spiral form through the air distributor on the top of the spray dryer to form a rotating hot air flow. The temperature and flow rate of hot air can be adjusted according to different drying needs.

After the atomized raw material liquid enters the drying tower, it is fully mixed and in contact with the rotating hot air flow. This is a heat and mass transfer process, in which hot air rapidly evaporates the moisture of the misty material, thereby achieving drying. The contact mode, mixing and flow state between droplets and air depend on the structural type of the hot air distributor, the installation position of the atomizer in the dryer, and the exhaust gas discharge method.

Under the action of hot air flow, the moisture in the droplets evaporates rapidly, causing the material to gradually dry. During this process, heat transfer and moisture diffusion and migration occur simultaneously inside the material, with surface vaporization and internal diffusion occurring simultaneously. As water evaporates, the material gradually turns into powder or fine particles.

The drying process can be divided into a constant speed drying stage and a reduced speed drying stage. In the constant speed drying stage, the speed of drying mainly depends on the evaporation rate of surface moisture of the material, which is controlled by external conditions. As the drying process progresses, the moisture content of the material gradually decreases and enters the slow drying stage, where the drying rate is mainly affected by the diffusion rate of moisture inside the material.